[$] Memory passthrough for virtual machines

Memory management is tricky enough on it own, but virtualization addsanother twist: now there are two kernels (host and guest) managing the samememory. This duplicated effort can be wasteful if not implementedcarefully, so it is not surprising that a lot of effort, from both hardwareand software developers, has gone into this problem. As Pasha Tatashinpointed out during a memory-management-track session at the 2023 Linux Storage, Filesystem, Memory-Managementand BPF Summit, though, there are still ways in which these systems runless efficiently than they could. He has put some effort into improvingthat situation.